It is often important for a device or an equipment having a movable body to ascertain the position of the movable body to perform control or operation. For example, some cameras such as a digital camera, a video camera or the like are constructed such that a zoom lens (lens-barrel) is moved by a motor when the focal length and the magnifying power of a lens system are changed. A reversibly rotatable motor is used as the motor. However, the motor itself is not provided with a positional information and thus a position detecting device for detecting the position of a zoom lens is separately provided (for example, see Japanese Patent Laid-Open No. 2001-324664).
The conventional position detecting device detects the position of a lens-barrel provided with the zoom lens and includes a light-shielding plate and two photo interrupters. The lens-barrel is mounted on the main body of a camera in a movable manner in the optical axis direction and is moved by a motor through a transmission means or the like. The focal length of a photographic optical system varies according to the movement of the lens-barrel.
The transmission means that moves the lens-barrel is a deceleration gear mechanism comprised of transmission wheels such as a gear. A shutter blade having three blades as the light-shielding plate is mounted on one of the gears constructing the deceleration gear mechanism. Three blades are disposed at 120 degree interval and two photo interrupters are disposed so as to be opposed to the three blades. The photo interrupter comprises of a light-emitting element and a light receiving element. The photo interrupter detects the passage of the blade between the light-emitting element and the light receiving element. A detected output signal is converted into a pulse and the number of revolutions of the motor is detected by counting the converted pulses. Accordingly, the position of the lens-barrel is detected by detecting the number of revolutions of the motor.
The two photo interrupters are disposed at 150 degree interval to detect the direction of rotation of the rotation shaft of the motor. When the motor is driven so that the shutter blade rotates in one direction, the output waveform of one of the photo interrupters leads ahead that of the other photo interrupter by a ¼ period. When the motor is driven so that the shutter blade rotates in the reverse direction, the output waveform of the one of the photo interrupters lags behind that of the other photo interrupter by a ¼ period. As a result, the difference of the output relations in the two photo interrupters is generated depending on the rotating direction of the shutter blade, that is, the rotation shaft of the motor, and therefore the rotating direction of the motor can be detected according to the difference. Consequently, the position of the lens-barrel can be accurately detected.
Cameras such as a digital camera or video cameras have been especially attempting to make their sizes smaller in recent years. A position detecting device equipped on the camera, the video camera or the like is also required to be made smaller. However, since the position detecting device is constructed such that the position of the lens-barrel is detected by using the shutter blade and the photo interrupter, a space for mounting the shutter blade and the photo interrupter, especially a space for mounting the photo interrupter is required, which causes the position detecting device to be difficult to downsize.
Also, in order to accurately detect the number of revolutions of the motor, the shutter blade is preferably arranged on the rotation shaft of the motor and the photo interrupter is disposed in the vicinity of the rotation shaft. However, a little vacant space is left in the vicinity of the rotation shaft of the motor for arranging the shutter blade and the photo interrupter and thus downsizing is not attained. Further, when two photo interrupters are used to accurately detect the position of the lens-barrel, a large space for disposing them is required and the number of component parts increases which further prevents downsizing.